Control system for servo valve and control method therefor, and combined-cycle electric-power generating plant and gas turbine plant

ABSTRACT

An object of the present invention is to provide a control system and a control method for servo valve which can stably continue to control a servo valve when a break occurs in servo coils of a servo valve having a plurality of servo coils. 
     In the present invention, voltages of a plurality of servo coils  3 A,  3 B,  3 C are detected by wire break detecting means individually provided to servo amplifiers  2 A,  2 B.  2 C. The servo amplifiers  2 A,  2 B.  2 C performs wire break detection and wire break compensation.

This application is a division of application Ser. No. 09/793,201 filedFeb. 26, 2001.

BACKGROUND OF THE INVENTION

The present invention relates to a control system for servo valve whichcontrols opening degree of the servo valve having a plurality of servocoils used in the field of chemical plant or electric-power generatingplant and a control method for the servo valve, and a combined-cycleelectric-power plant and a gas turbine plant having the control systemfor servo valve.

DESCRIPTION OF THE RELATED ART

In general, in the field of chemical plant or electric-power generatingplant, a servo valve having a plurality of servo coils is used as avalve for controlling an important plant amount. Even if a break occursin one of the servo coils, the servo valve having a plurality of servocoils can control the servo valve by compensating the break in the servocoil using the other sound servo coils, and this can prevent a shutdownor a power swing of the plant, which in turn can improves thereliability of the operation.

In order to perform the compensation of servo coil break, it isnecessary to detect occurrence of a break in the servo coils. In thepast, detection of wire break in the servo coil has been detected by acontrol operation unit such as a microcomputer outputting a servocontrol voltage demand based on an opening degree difference between atarget opening degree and a real opening degree of the servo valve. Thistechnology is disclosed, for example, in Japanese Patent ApplicationLaid-Open No.9-126351.

In addition, there is proposed a method in which a break detectingresistor is connected to a plurality of servo coils in common, and abreak in the servo coil is detected by a voltage of the break detectingresistor.

Since the technology disclosed in Japanese Patent Application Laid-OpenNo.9-126351 detects a break in servo coils using the control operationunit such as a microcomputer, the detection is dependent on theoperation cycle, and accordingly there is a problem in the high speedoperability. Since the operation cycle (sampling cycle) is 50 ms to 100ms, the technology can not cope with the requirement of 5 ms.

On the other hand, the technology of using the common break detectionresistor to the plurality of servo coils has a problem in that when afailure occurs in the common break detection line, the function of breakdetection is lost.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a control system and acontrol method for servo valve which can stably continue to control aservo valve when a break occurs in servo coils of a servo valve having aplurality of servo coils, and to provide a combined-cycle electric-powergenerating plant and a gas turbine plant having the control system forservo valve.

The present invention is characterized by that a break in a servo coilis detected by a voltage in each of a plurality of servo coils.

A preferred embodiment of the present invention is that detection ofwire break in the servo coil and compensation of wire break areperformed by servo amplifiers.

That is, since a break in the servo coil is detected by a voltage ineach of a plurality of servo coils, the break can be instantaneouslydetected. Further, since the compensation of wire break in the servocoil is performed by sharing the load current of the broken servo coilamong the unbroken servo coils, the control of servo valve can be stablycontinued.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an embodiment in accordance with thepresent invention.

FIG. 2 is a detailed block diagram showing an example of themicrocomputer of FIG. 1.

FIG. 3 is a detailed block diagram showing an example of the servoamplifier of FIG. 1.

FIG. 4 is a graph explaining operation of the present invention.

FIG. 5 is a graph explaining operation of the present invention.

FIG. 6 is a graph explaining operation of the present invention.

FIG. 7 is a graph explaining operation of the present invention.

FIG. 8 is a graph explaining operation of the present invention.

FIG. 9 is a graph explaining operation of the present invention.

FIG. 10 is a graph explaining operation of the present invention.

FIG. 11 is a graph explaining operation of the present invention.

FIG. 12 is a system diagram showing a combined-cycle electric-powergenerating plant to which the control system foe servo valve inaccordance with the present invention is applied.

FIG. 13 is a system diagram showing a gas turbine plant to which thecontrol system foe servo valve in accordance with the present inventionis applied.

DETAILED DESCRIPTION

An embodiment of the present invention is shown in FIG. 1 to FIG. 3.FIG. 1 is a block diagram showing the embodiment in accordance with thepresent invention. FIG. 2 is a detailed block diagram showing an exampleof the microcomputer of FIG. 1. FIG. 3 is a detailed block diagramshowing an example of the servo amplifier of FIG. 1. Therein, thedetailed block diagrams of the examples shown be FIG. 2 and FIG. 3 arethe same structures for the microcomputers 1B, 1C and the servoamplifiers 2B, 2C, respectively.

In FIG. 1, the microcomputer 1A, the servo amplifier 2A and the servocoil 3A are connected so as to form one set, and the microcomputer 1B,the servo amplifier 2B and the servo coil 3B are connected so as to formone set, and the microcomputer 1C, the servo amplifier 2C and the servocoil 3C are connected so as to form one set. Although FIG. 1 shows theexample of three servo coils 3A to 3C, the present invention is notlimited to three servo coils, and can be applied to a case of two servocoils or a case of four or more servo coils.

The embodiment will be explained by taking the set of the microcomputer1A, the servo amplifier 2A and the servo coil 3A.

Real opening degrees AI1, BI1 of the servo valve 4 are input to themicrocomputer 1A shown in FIG. 2 from the opening meters 5A, 5B, and apractical opening value is selected by a high-value selector 7, and acontrol command based on a difference between a preset target value andan opening degree signal output from the high-value selector 7 isgenerated by a difference calculator 8, and then the control command istransmitted to the servo amplifier 2A by outputting it to an operationalamplifier 25 shown in FIG. 3.

Therein, in a switch (T1) 10 shown in FIG. 2, when a state D1 forstarting servo valve control is input from a control start relay 6 tobring the switch (T1) 10 in a starting state (the relay contact is ON),the output from the difference calculator 8 is selected to bring thesystem to the control mode state. When the control start relay 6 is OFFstate (the relay contact is OFF), the switch 10 selects an output of aconstant value generator 9 to keep the command to the servo valve 3A inan initial state, that is, a full-open or full-close state.

The command to the operational amplifier 25 is operated by a multiplier11 to be output as a servo control command. In the servo amplifier 2A,the operational amplifier 25 receiving the servo control commandtransmits the output to a voltage/current converter 26 to convert thevoltage to a current to be output.

A contact signal of the control start relay 6 is input to a switch 27,and the current is output to the servo coil 3A if the control startrelay 6 is in ON-state to turn the switch 27 ON.

Similarly, in the B system of the microcomputer 1B, the servo amplifier2B and the servo coil 3B, a current is output to the servo coil 3B bythe microcomputer 1B and the servo amplifier card 2B. In the C system ofthe microcomputer 1C, the servo amplifier 2C and the servo coil 3C, acurrent is also output to the servo coil 3C by the microcomputer 1C andthe servo amplifier card 2C.

Thus, the servo valve 4 is driven by the sum of the current signalsindividually output to the servo coil 3A, the servo coil 3B and theservo coil 3C.

Therein, the servo amplifiers 2A to 2C may be mounted on a single card.

FIG. 4 is shows an example of the characteristic of an operation resultexecuted by the microcomputer 1. The difference (mA) between the targetvalue (mA) and a current value corresponding to the detected openingdegree (%) value output from the high-value selector 7 is calculated bythe difference calculator 8, and the magnitude of the calculation resultof difference is output to the servo amplifier 2A as the servo controlcommand.

FIG. 5 shows an example of changes in current flowing in the servo coils3A to 3C and changes in total current flowing in the servo coils. Thedifferences obtained from calculations in the microcomputers 1A to 1Care input to the servo amplifiers 2A to 2C, and currents are out put tothe servo coils 3A to 3C, respectively. The servo coils 3A to 3C receivethe output currents having a current error of ±8 mA and a voltage of 1Vto 5V from the servo amplifiers 2A to 2C to drive the servo valve 4.That is, when the real opening value is equal to the target value, thecurrent outputs of the servo amplifiers 2A to 2C are 0, and the servovalve 4 is in balance by 3V (assuming that the resistance of each coilis 375Ω, a value of current flowing in it is 8 mA) per one servo coil 3(the total voltage value is 9V by the three coils 3A, 3B, 3C, and thetotal current value is 24 mA).

The detection of wire break and the compensation of wire break in a caseof a break in one servo coil will be described below.

Referring to FIG. 3, if a wire break occurs in the servo coil 3A, thewire break is detected by a wire break detection voltage monitoring part28A which monitors voltage of a current loop formed up to the servo coil3A by the current output from the voltage/current converter 26. That is,the wire break is detected by a voltage drop of the wire break detectionvoltage monitoring part 28A when the wire break occurs in the servo coil3A. The operational principle of the wire break detection by the voltagemonitoring of the current loop is obvious from the Ohm's law.

After the wire break detection voltage monitoring part 28A detects thewire break, an OR circuit 31 transmits a wire break detection signalexpressing the servo coil 3A being broken from the servo amplifier 2A tothe microcomputer 1A. In the microcomputer 1A, by the wire breakdetection signal of the OR circuit 31, a pickup timer 17 and a weighttimer 18 shown in FIG. 2 execute operation to memorize the first wirebreak in a flip-flop 22.

Although the case of occurrence of a wire break in the servo coil 3A isexplained above, in a case of occurrence of a wire break in the servocoil 3B or 3C the similar operation is performed by the microcomputer 1Bor 1C and the servo amplifier 2B or 2C.

In the case of occurrence of the wire break in the servo coil 3A, theswitches 14 of the microcomputers 1B, 1C select 1.5 as the constant fromthe constant value generators 12 by the output of the OR circuits 29 tomultiply the servo control command by 1.5 using the multipliers 11. Thatis, in a case where a wire break occurs in one of the servo coils 3A to3C (in this case, 3A), the control command is increased to 1.5 times ofthe normal value in the microcomputers 1.

In the servo amplifier 2A, the output of the weight timer 45 of thefirst wire break is memorized in a counter 46 (the first wire breakis 1) to hold a monitor relay 47. The operation of the compensation ofwire break is performed by selecting the gain 1.5 of the constant valuegenerator 40 using the switch 43 and by multiplying the output of theoperational amplifier 25 by 1.5 using the multiplier 49.

Therein, FIG. 6 shows the control characteristics in the case where thewire break occurs in the servo coil 3A. FIG. 7 shows the characteristicsof the operation of voltage signal in the servo amplifier 2A and theoutput to the servo valve 4 at compensating the wire break.

It is clear from FIG. 7 that the output of the servo coil 3A isdecreased because the wire break occurs in the servo coil 3A, but theoutputs of the servo coil 3B and the servo coil 3C are increased by thegain of 1.5, and accordingly the output of the servo valve 4 is notchanged and kept in a state similar to the state of three coils alive.That is, as shown in FIG. 6, by increasing values of the respectivecontrol commands for the servo coils 3B and 3C to 12 mA, the openingdegree (%) of the servo valve 4 can be kept constant.

Next, it is assumed that a wire break occurs in the servo coil 3B undera state where control is being performed using the servo coil 3B and theservo coil 3C. In this case, the wire break is detected by the wirebreak detection voltage monitoring part 28B of the servo amplifier 2B,similarly to the wire break of the servo coil 3A. After the wire breakdetection voltage monitoring part 28B detects the wire break, the ORcircuit 31 shown in FIG. 3 transmits a wire break detection signalexpressing occurrence of the second wire break of the three coils fromthe servo amplifier 2B to the microcomputer 1B.

In the microcomputer 1B, by the wire break detection signal of the ORcircuit 31 of the servo amplifier 2B, the flip-flop 33 memorizing thefirst wire break and a time pickup 23 holds an AND circuit 30. Theswitch 16 selects 3.0 from the constant value generator 15 to multiplythe servo control command by 3.0 using the multiplier 11.

FIG. 8 and FIG. 9 show the control characteristics in the case where thewire break further occurs in the servo coil 3B. At occurring of the wirebreak in the servo coil 3B, the control command is increased to 3.0times of the normal value in the microcomputers 1, and at the same timethe second wire break is memorized in the counter 46 (the second wirebreak is 2) in the servo amplifier 2B. Further, the switch 44 selectsthe gain 3.0 of the constant value generator 41 by holding of a monitorrelay 48. The servo control command of the servo amplifier 25 ismultiplied by 3.0 by the multiplier 49 to perform operation of thecompensation of the wire break. It can be understood from FIG. 9 thatthe output of the servo coil 3B is decreased because of occurrence ofthe wire break, but the output of the servo coil 3C is increased by thegain of 3.0, and accordingly the total output of the servo valve 4 isnot changed. That is, as shown in FIG. 8, by increasing the value of thecontrol command for the servo coil 3B to 24 mA, the opening degree (%)of the servo valve 4 can be kept constant.

In order to perform a rapid closing (or rapid opening) operation of theservo valve 4, the output of the servo coil 3C is changed to 0. That is,the control start relay 6 in the microcomputer 1C is switched OFF. Sincethe switch (T1) 10 selects 0 of the output of the constant valuegenerator 9, the servo control command of the microcomputer 1 rapidlyfalls to 0, as shown in FIG. 10. Therefore, the servo amplifier 2Cgenerates a fully-closing output, and the servo valve 4 is rapidlyclosed, as shown in FIG. 11.

The servo valve is controlled as described above. Since a break in theservo coil is detected by a voltage in each of a plurality of servocoils, the break can be instantaneously detected. Further, since thecompensation of wire break in the servo coil is performed by sharing theload current of the broken servo coil among the unbroken servo coils,the control of servo valve can be stably continued.

Further, in the above embodiment, when operation of rapid closing orrapid opening of the servo valve is required, the operation time of theservo valve can be shortened by directly inputting the condition to theservo amplifier. Therein, at that time when the servo valve is fullyclosed or fully opened, the load current applied to the servo coils doesnot continue to be supplied to the servo coils, but is interrupted.Therefore, the life time of the coils can be extended.

As described above, since a break in the servo coil is detected by avoltage in each of a plurality of servo coils, the break can beinstantaneously detected. Further, since the compensation of wire breakin the servo coil is performed by sharing the load current of the brokenservo coil among the unbroken servo coils, the control of servo valvecan be stably continued.

Description will be made below on a case where the control system forservo vale described above is applied to a combined-cycle electric-powerplant.

FIG. 12 is a system diagram showing an embodiment of a combined-cycleelectric-power generating plant. Referring to FIG. 12, thecombined-cycle electric-power generating plant comprises a gas turbineunit composed of a compressor 50 for compressing air, a combustor 51 forburning a gasified fuel such as LNG or the like, and a turbine 52 whichis driven by burned gas obtained by mixing the air compressed by thecompressor 50 and the burned gas burned by the combustor 51; a generator53 linked to the gas turbine; an exhaust heat recovery boiler 54 forgenerating steam using high temperature exhaust gas exhausted from thegas turbine 52; and a steam turbine 55 driven by the generated steamfrom the exhaust heat recovery boiler 54, and further comprises acondenser for condensing steam from the steam turbine 55 and a pump 57for feeding condensate water to the exhaust heat recovery boiler 54.

In the combined-cycle electric-power plant shown by FIG. 12, the gasturbine system and the steam turbine 55 are coaxially linked to thegenerator 53 in order to make the plant compact. However, there are somecases where the gas turbine system and the steam turbine 55 areseparately linked to individual generators.

In the combined-cycle electric-power plant, a servo valve 4A is placedin a pipe for conveying the gasified fuel such as LNG or the like to thecombustor 51. The servo valve 4A is controlled by a gas turbine controlunit 58, but the gas turbine control unit 58 is constructed so as toinclude the control system for servo valve described in FIG. 1 to FIG.3.

Similarly, servo valves 4B, 4C, 4D are individually placed in pipesconveying generated steam from the exhaust heat recovery boiler 54 tothe steam turbine unit 55. The servo valves 4B, 4C, 4D are controlled bya steam turbine control unit 59, but the steam turbine control unit 59is constructed so as to include the control system for servo valvedescribed in FIG. 1 to FIG. 3.

By applying the servo valve control system as described above, since abreak in the plurality of servo coils exciting to operate closing andopening each of the servo valves 4A to 4D is detected by a voltage ineach of a plurality of servo coils, the break can be instantaneouslydetected. Further, since the compensation of wire break in the servocoil is performed by sharing the load current of the broken servo coilamong the unbroken servo coils, the control of servo valve can be stablycontinued. Therefore, the combined-cycle electric-power generating plantcan be stably operated.

Description will be made below on a case where the control system forservo vale described above is applied to a gas turbine plant.

FIG. 13 is a system diagram showing an embodiment of a gas turbineplant. Referring to FIG. 13, the gas turbine plant comprises acompressor 59 for compressing air; a combustor 60 for burning a gasifiedfuel such as LNG or the like; a turbine 61 which is driven by burned gasobtained by mixing the air compressed by the compressor 59 and theburned gas burned by the combustor 60; and a generator 62 linked to thegas turbine 61.

In the gas turbine plant, a servo valve 4E is placed in a pipe forconveying the gasified fuel such as LNG or the like to the combustor 60.The servo valve 4E is controlled by a gas turbine control unit 63, butthe gas turbine control unit 63 is constructed so as to include thecontrol system for servo valve described in FIG. 1 to FIG. 3.

By applying the servo valve control system as described above, since abreak in the plurality of servo coils exciting to operate closing andopening the servo valve 4E is detected by a voltage in each of aplurality of servo coils, the break can be instantaneously detected.Further, since the compensation of wire break in the servo coil isperformed by sharing the load current of the broken servo coil among theunbroken servo coils, the control of servo valve can be stablycontinued. Therefore, the gas turbine plant can be stably operated.

What is claimed is:
 1. A method for controlling a servo valve with acontrol system, the servo valve having a plurality of servo coils, theservo valve being excited by said servo coils, the method comprising:outputting a servo control voltage command based on an opening degreedifference between a target opening degree and a real opening degree ofsaid servo valve; converting said servo control voltage command tocurrent to excite said plurality of servo coils to a predeterminedcurrent value and to perform compensation of servo coil breaks;detecting a break in one of said servo coils by detecting a change ofvoltage of each of said plurality of servo coils; and increasing theservo control voltage command to compensate for the detected break inone of said servo coils.
 2. A method for controlling a servo valve witha control system according to claim 1, comprising increasing a gain ofthe servo-control voltage command, when a wire break is detected.
 3. Amethod for controlling a servo valve with a control system according toclaim 1, wherein the compensation of a servo coil break is performedbased on detecting a break in any servo coil among said plurality ofservo coils.
 4. A method for controlling a servo valve with a controlsystem according to claim 1, wherein said servo control voltage commandis multiplied by a predetermined constant based on detecting a break inany of said servo coils.
 5. A method for controlling a servo valve witha control system according to claim 1, wherein the servo control voltagecommand is generated by a microcomputer.
 6. A method for controlling aservo valve with a control system according to claim 1, whereinconverting the servo control voltage command is performed by a servoamplifier which includes a function of voltage monitoring for detectinga wire break.
 7. A method for controlling a servo valve with a controlsystem according to claim 1, wherein converting the servo controlvoltage command is performed by a servo amplifier which amplifies thevoltage command from the controller and then converts the amplifiedvoltage into a current.
 8. A method for controlling a servo valve with acontrol system, the servo valve having three servo coils, the servovalve being excited by said servo coils, the method comprising:outputting a servo control voltage command for each of said servo coilsbased on an opening degree difference between a target opening degreeand a real opening degree of said servo valve; converting said servocontrol voltage command to current to individually excite each of saidthree servo coils to a predetermined current value and to performcompensation of servo coil breaks; detecting a break in one of saidservo coils by detecting a change of voltage of each of said three servocoils; and increasing the servo control voltage command to compensatefor the detected break in one of said servo coils.
 9. A method forcontrolling a servo valve with a control system according to claim 8,wherein the compensation of a servo coil break is performed based ondetecting a break in any servo coil among said three servo coils.
 10. Amethod for controlling a servo valve with a control system according toclaim 8, wherein said servo control voltage command is multiplied by apredetermined constant based on detecting a break in any servo coilamong said three servo coils.
 11. A method for controlling a servo valvewith a control system according to claim 8, wherein the servo controlvoltage command is generated by a microcomputer.